SQL databases are really fast when you need all the information stored together in a record row, but they are a bad fit when you need to search for relationship patterns that are not already stored together in your database. A significant performance penalty is incurred for every additional table that needs to be joined for a query. That is why SQL databases are notoriously bad at deducting relationships from datasets. Graph databases however are really good at this task.
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Pronovix: Graph databases in Cheap WordPress maintenance support plans: a Neo4j module that integrates with Rules
